Hydrogel-forming polymers are those which are not water soluble, but which upon contacting aqueous fluids, imbibe such fluids. Upon absorbing water, the mechanical properties of articles formed from such polymeric hydrogel materials change. For example, articles which are rigid when dry may become soft and pliable when wet.
Certain of these systems are used in commerce ranging from use as “supersorbers” in baby diapers to more sophisticated uses in the manufacture of soft contact lenses. Polymer hydrogel systems include ionically linked hydrogels, covalently crosslinked hydrogels, and the like.
Ionic hydrogels are polymers containing functional groups that ionically bond to form a crosslinked system. Such materials are used to form toy products that can be continuously reformed and reshaped. These materials, however, cannot be formed into a permanent shape or configuration.
Covalently crosslinked hydrogels are formed by crosslinking otherwise water-soluble polymers. Once these materials are crosslinked, they will no longer dissolve in water, or in any other solvent for that matter. Nevertheless, these crosslinked materials will absorb water and swell in dimension. Compositions such as crosslinked poly-2-hydroxyethylmethacrylate are used to form contact lenses. These materials, however, cannot be thermally processed.
Mechanically crosslinked systems are formed when two polymers which are incompatible are chemically linked as either block copolymers or graft copolymers. On the one hand, if the two incompatible homopolymers are simply blended together, a gross phase separation occurs. If on the other hand, the two polymers or connected together covalently, a microphase separation occurs.
This microphase separation is a well known phenomenon that has been utilized, most notably, in the styrene-butadiene Kraton triblock rubber field. Block copolymers partition into different phases, a continuous phase consisting of the higher percentage polymer and a discontinuous or dispersed phase consisting of the lower percentage polymer. The dispersed phase can form very distinct domains, including spheres, cylinders, and lamella. The configuration of these domains is related to the relative percentages of the two phases. At low percentages, the dispersed phase comprises spherical domains. As that percentage increases the dispersed phase forms cylindrical domains and finally discrete lamella.
Significantly, a microphase separated two-component polymeric material exhibits two glass transition temperatures (tg's), namely one for each polymer present. When the temperature of the composite material is below the respective glass transition temperatures of both components, the copolymer is rigid. If the composite material is warmed above the glass transition temperatures of both phases, the copolymer will flow. Therefore, such a mechanically crosslinked copolymer can be thermally processed using conventional molding techniques such as extrusion and injection molding.
A mechanically crosslinked hydrogel copolymer comprises a block or graft copolymer formed from incompatible polymers where the continuous phase is a water soluble polymer. Various water soluble organic polymers prepared from substituted ethylenes are known, including polyvinylpyrrolidone, polyvinylalcohol, and the like. Polyethylenimine, and certain of its derivatives is also water soluble.
Poly-2-alkyl-2-oxazolines, where the alkyl group comprises 1-5 carbons, are such water soluble, N-acyl polyethyleneimines. Significantly, these poly-2-alkyl-2-oxazolines have thermal properties that differ materially from water soluble polymers prepared from substituted ethylenes. For example, poly-2-ethyl-2-oxazoline has both a low dry glass transition temperature of 70-71° C. and a high thermal degradation temperature above 380° C.
Prior art mechanically crosslinked hydrogel systems do not include systems based upon poly-2-alkyl-2-oxazolines. U.S. Pat. No. 4,582,877, in the name of Fairchok et al., teaches a method to enhance the wettability of polypropylene by blending polypropylene with substituted polyethyleneimines. Fairchok et al. teaches preparation of those substituted polyethyleneimines by reacting a poly-2-oxazoline with a carboxylic acid to form a polyethyleneimine having pendent groups of up to 22 carbon atoms. Col. 1/1. 50 to Col. 2/1. 17. Each such 22 carbon pendent side group would increase the molecular weight of the polyethyleneimine by only about 300 grams/mole. In a most preferred embodiment, Fairchok et al. teaches preparation of a covalently crosslinked material by reacting a poly-2-oxazoline with a polybasic carboxylic acid. Col. 2/1. 18-25.
In contrast, Applicant's invention comprises a graft copolymer comprising a water soluble poly-2-alkyl-oxazoline having a plurality of pendent non-water soluble polymers grafted thereon, wherein each of the pendent non-water soluble polymers has a number average molecular weight of at least 5,000. Moreover, Applicants' invention does not encompass any covalently crosslinked systems.